Noun
English Wikipedia has an article on:polyominoWikipedia
polyomino (plural polyominos or polyominoes)
(geometry) A plane polyform made by joining one or more squares edge to edge in various arrangements; a similar polyform made from triangles or hexagons (or any polygon that can tile a plane)
A free tetromino is a free polyomino made from four squares. Source: Internet
But it uses polyomino from 1-block monomino to 5-block pentomino that fit at most 5x5 box. Source: Internet
It is a polyomino with five cells. Source: Internet